Current approaches to robust operation in the presence of severe environmental stresses include specifying high immunity levels, additional filtering/suppression devices and watchdog timers.
High electrostatic discharge (ESD) immunity levels are typically specified for all parts. However, the ESD immunity levels do not provide robust operation in the presence of all stress conditions. Additionally, no standard immunity test for electrical test transient/burst (EFTB) (or similar) events exists, since the events depend on particular device implementation. The addition of extra devices to increase immunity (i.e., add bypass capacitance to relevant signals) has the disadvantage of extra cost and space penalties.
Watchdog reset circuits (i.e., to reset a device that has left normal operating mode) allow recovery from a known fault condition caused by overstress events. Watchdog reset circuits are most effective in cases of significant failure that places the device in a recognized fault mode (i.e., program code may be vectored to an illegal location, which causes code execution to halt until the watchdog reset occurs).
However, many cases exist where a stress condition causes faulty operation that is not recognizable by the watchdog reset circuit. EFTB testing shows many instances where a device stops operating properly but fails to trip the watchdog reset. In one example, the device may vector to an unintended part of code, resulting in faulty operation even though the device continues to operate within legal portions of the code. In another example, corruption of memory (i.e., RAM) can occur during an event, but go undetected and cause failure at a later time. The watchdog reset circuit is unable to detect when such problems occur.
Specifying immunity levels (i.e., 2 kV ESD) for recovery of stress conditions of devices increases cost and does not prevent all stress condition related problems. Adding (off-chip) filtering/suppression devices increases cost and has related space concerns. Additionally, watchdog timer circuits can enter confused modes that will not trip the watchdog circuit.
It is generally desirable to provide proper circuit operation during and/or after stress events (i.e., ESD, noise transients, etc.)